In general, automobile exhaust gas is described by a gas that is discharged to the atmosphere through an exhaust pipe after a gasoline-air mixture is combusted in an engine. This exhaust gas includes harmful materials such as mainly carbon monoxide (CO), nitrogen oxide (NOx), un-combusted hydrocarbon (HC) and the like. Emission and exhaust regulations for controlling such exhaust gas become stricter and stricter. Accordingly, exhaust gas control devices such as an exhaust gas recirculation device, a tertiary catalyst, a MPI device and the like, and vapor gas control devices such as a canister, a purge control solenoid value and the like are applied to automobiles.
On the other hand, in case of automobile equipped with a diesel engine, the exhaust gas includes a considerable amount of nitrogen oxides and particulate matters, rather than the case of a gasoline engine, notwithstanding that the diesel engine has a better fuel efficiency and horsepower. Since most of air is combusted enough under operational conditions of diesel engines, the diesel automobile exhausts a less amount of CO and HC, rather than in the gasoline engine. However, the diesel engine exhausts a larger amount of NOx and particulate matters (fume).